Molecular sieves are used in numerous commercial processes for hydrocarbon conversion and/or separation, particularly the conversion and/or separation of aromatic hydrocarbons, such as transalkylation, disproportionation, isomerization, adsorptive separation, and the like. Very often the goal is to produce more xylenes from one or more of benzene, toluene, and aromatic hydrocarbons having 9 carbons or more (C9+ aromatic hydrocarbons or more simply “A9+”). For example, the production of xylenes via transalkylation of A9+ with C6 and/or C7 aromatics to produce xylenes may use a molecular sieve based catalyst such as at least one of Mordenite, ZSM-12, MCM-22 family material, and combinations thereof. Certain molecular sieves are useful to purify one or more of the feed streams upstream of the transalkylation reaction and certain molecular sieves may also be useful in separation and/or isomerization of the xylene product downstream of the transalkylation reaction.
Typically molecular sieves lose performance, such as activity, selectivity, and capacity, through various deactivation mechanisms. As the molecular sieve catalyst or adsorbent ages with increasing time on stream, more severe conditions, such as higher temperature and/or low through-put, are normally required to maintain comparable activity and/or selectivity. When the maximum reactor temperature and/or minimum acceptable through-put is reached, the molecular sieve catalyst or adsorbent needs to be replaced or regenerated/rejuvenated. The spent catalyst, at the end of its useful life, may contain a significant amount of coke, often exceeding 35 wt %, and sometimes even as high as 50 wt %. Various techniques, sometimes called regenerative or rejuvenative, are known that remove the deposited coke, in whole or in part, and allow the molecular sieve to be reused. These techniques may be performed in situ and/or ex situ, depending on the technique and other factors.
One common regeneration technique is to burn the coke from the molecular sieve in an oxidative environment, such as air or oxygen. It had been thought that the oxidative calcination process normally needed to be controlled with dry air to prevent steam damage to the molecular sieve framework, for example, steam dealumination of zeolite, which causes severe damage to the molecular sieve. Recently it was discovered that the controlled addition of water during the regeneration, combined with a staged controlled temperature burn, can successfully regenerate a heavily coked catalyst with minimal steam damage of the molecular sieve structure. See U.S. application Ser. No. 12/738,057.
Another less common regeneration technique is to rejuvenate a spent catalyst in a reductive environment, such as hydrogen. However, if the catalyst is not regenerated properly, aging rates during the second cycle can be very high resulting in a second cycle length as short as less than 10% of the first cycle length.
Other regeneration techniques include the use of steam or other solutions in combination with heating or calcining. For example, U.S. Pat. No. 5,093,293 discloses the use of steam for removing coke and other contaminants from Zeolite L, and U.S. Pat. No. 4,139,433 discloses that a hydrocracking catalyst containing a Group VIII metal is regenerated by treating the spent catalyst with an ammonium hydroxide solution followed by calcination. Still other techniques are disclosed in U.S. Pat. Nos. 4,975,399 and 4,550,009. There are no doubt a plethora of additional techniques and the aforementioned citations are intended merely to be representative thereof.
In typical commercial operations molecular sieve catalysts usually have a finite cycle time, which is the length of time over which the process is operated. The cycle time is usually determined by when the temperature required to maintain constant conversion to offset the declining activity exceeds the physical limitations of the equipment, or it may be determined by some other indicia of efficiency, such as through-put. As the catalyst approaches a certain temperature, the aging rate exponentially increases such that it is no longer efficient (e.g., viable and/or economical) to maintain operations.
The present inventors have noted that this correlates with the amount of coke on the catalyst; the more coke on the catalyst, the closer the catalyst is to reaching the defined cycle length. In addition, the present inventors have noted that this also correlates with the extent to which the coke is graphitic in nature, e.g., coke having a relatively low H/C ratio. The catalyst then needs to be regenerated or replaced. Given the cost of a new catalyst load, the preferred method of choice to regain catalyst activity is to regenerate/rejuvenate the catalyst, either in-situ or ex-situ, to remove the coke that has built up on the catalyst.
The present inventors have further discovered that the end-of-cycle condition of the molecular sieve catalyst with respect to wt % coke and/or the graphitic nature of the coke, as measure by H/C ratio, is an important factor in determining subsequent cycle performance after regeneration, and thus a better indicia of the appropriate cycle time, rather than indicia used in the prior art.